1. Field of the Invention
The invention relates in general to drum brakes, and more specifically to spring-applied, electrically released drum brakes.
2. Description of the Prior Art
Escalators are provided with an electrically released, mechanically applied brake capable of stopping an up or down traveling escalator with any load up to brake design load.
The maximum braking effort is required to stop a fully loaded escalator going down, and thus the brake is sized accordingly. For example, the brake torque is selected to provide some minimum value of deceleration, such as about 1 ft/sec.sup.2, when an escalator with rated load is stopped while transporting passengers from an upper landing to a lower landing. Thus, any other condition than a fully loaded escalator going down will result in a higher rate of deceleration. The highest rate of deceleration would occur when a fully loaded escalator is braked to a stop while transporting passengers from the lower landing to the upper landing. This may be about 8 to 10 ft/sec.sup.2 for a typical escalator with a fixed braking torque.
The prior art has disclosed many different arrangements which adjust the braking effort, in order to decrease the range between the minimum and maximum rates of deceleration which may occur, by taking such things as speed, load, and/or travel direction into account. For example, the braking effort may be adjusted according to load, according to speed, such as in response to an error signal which is responsive to the difference between the actual speed and the desired speed of the escalator while braking to a stop, or in response to travel direction. In general, such controlled braking arrangements add substantially to the cost of an escalator, as well as to the maintenance thereof, because of the more complex mechanical and/or electrical apparatus required.
It would be desirable to provide a new and improved brake suitable for an escalator which will inherently provide substantially less braking force in one direction than the other, enabling such a brake to be used to inherently brake with a greater force when the escalator is moving such that it would transport passengers from an upper to a lower landing.
It would further be desirable to provide a new and improved brake with the above-mentioned inherent directional braking effort capabilities, which will apply the braking force within predetermined jerk and deceleration constraints, with a deceleration rate which is substantially constant, regardless of load, for a down traveling escalator.